Method and solution for treating aluminum



United States Patent 2,105,191 METHOD AND SOLUTION FOR TREATING ALUMINUMJohn F. Jumer, Chicago, Ill.

No Drawing. Application October 31. 1949, Serial No. 124,730

12 Claims. (CI. 41-42) My invention pertains to the, art of chemicalpolishing, and particularly to the so-called bright dip solutionsadapted to impart a mirror or satin finish to articles formed ofaluminum. 1

Aluminum and a large majority of the commercial aluminum alloys aresubject to the formation of a gray, unattractive surface film, whichtenaciously retains grease and dirt, and which is conventionally removedby mechanical, or in some instances, by electro-chemical polishing. Inmany instances, the surface of the article upon which film forms isquite smooth and free from blemishes, as is frequently the case inregard to articles formed on a punch press. Since mechanical smoothingor abrading of these surfaces is unnecessary, attempts have been made toeliminate the film by treating the surface of the article with asolution adapted'to dissolve the film. A reasonable degree of successhas been experienced in the use of some of these solutions, but each haspossessed one or more objectionable characteristics, such as amarkedtendency to attack and etch the metal surface, to disassociatedalloy constituents, to generate noxious fumes, or other similarshortcomings.

The primary object of my invention is to provide a solution whichovercomes these defects, and which removes the objectionable surfacefilm from articles formed of aluminum or aluminum alloys containing lessthan five per cent silicon, leaving on the surface of the article eithera highly lustrous mirror finish or a satin finish at the option of theoperator.

Other objects of my invention, which include the provision of aninexpensive solution for polishing aluminum, and a satisfactory methodfor its utilization, will be disclosed in the course of the followingdescription.

In its preferred form, my solution, or bath, is formed by adding to 100(75%) about 6 grams of sodium nitrate and 1.5 grams of sodium sulphate.If a mirror finish is desired, the temperature of this solution ismaintained at approximately 210-220 F., and the article immersed in thebath for -180 seconds, care being taken to avoid entrapment of airagainst the surface of the article. Upon removal of the article from thesolution, it will be covered with a gelatinous film, which may beremoved if desired by scrubbing, but which is more convenientlyeliminated by dipping the article in a dilute aqeuous solution of nitricacid. This treatment completely removes the gray, unattractive film fromthe surface and produces a very high luster, superior to a vast majorityof similar articles produced by mechanical means.

The above-described process and composition is satisfactory for allcommercial alloys of aluminum, with the exception of those containingmore than five per cent silicon. When the silicon content exceeds fiveper cent, the polishing action of the solution is irregular, which I/donot consider desirable.

The various items employed in compounding my bath are of commercialgrade, the phosphoric acid being that known to the trade as 75 per cent,meaning thereby that the water content of the solution is approximatelyper cent. Certain other acids, or acid anhydrides, pure or mixed, ofphosphorous may be employed, as for example 85 per cent orthophosphoricacid, polyphosphoric acid, or phosphorous peritoxide, but when employedshould be diluted to yield a concentration approximately equal to thatobtained by utilizing-75 per cent phosphoric acid. The various alkalimetal nitrates and sulfates may preferentially attack certain partiallycc. of commercial phosphoric acid in general be substituted for thesodium nitrate and sodium sulfate, if desired, allowance being made forthe difference in molecular weight; but this is generally unnecessaryand undesirable, since such salts are somewhat more costly than thosespecified. The concentration of the salts may also be varied withincertain limits from the optimum specified. Satisfactory results will beobtained when a concentration of sodium nitrate per cc.- of 75 per centphosporic acid lies between 0.5 and 30.0 grams. Similarly, theconcentration of sodium sulfate per 100 cc. of'acid may vary from 0.25grams to 10.0 grams, or in other words, an approximate nitrate tosulfate ratio of 3.1.

' When the operator wishes to attain a mirror finish, the temperature ofthe solution should be maintained above 180 F. and below 275 F. Withinthis temper-, ature range, the higher temperatures tend to reduce, theimmersion time required to attain a bright surface finish, but also tendto generate objectionable fumes, which increase in volume with thetemperature. If the temperature of the-bath is maintained between andI80- F., a satin, rather than a mirror finish will be produced on thearticle. The length of time the solution must be left in contact with.the surface being treated varies with the temperature ofthe, solutionand the thickness and character of the film, but in general asatisfactory mirror finish can be produced within from 1 to 180'secends, 20 to 60 seconds representing the normal optimum and averagetime of treatment. A satin finish is usually attained in from 5 to 50seconds.

It is usually most convenient to treat the surface of the articlebyimmersing the article in the bath or solution. However, the solutionmay be swabbed on the surface, sprayed, or applied in any other mannerdesired, provided the concentrations and temperatures prescribed aremaintained within their operating ranges.

I claim:

1. An aqueous solution for chemically polishing articles formed of ametal consisting principally of aluminum and containing not more than 5per cent silicon comprising principally aqueous phosphoric acidcontaining 0.5-30.0 grams of an alkali metal nitrate and 0.25-10.0 gramsof an alkali metal sulfate per 100 cc. of phosphoric acid.

2. An aqueous solution for chemically polishing articles formed of ametal consisting principally of aluminum and containing not more than 5percent silicon comprising a concentrated aqueous phosphoric acidsolution containing 0.5-30.0 grams of sodium nitrate and 0.25-10.0 gramsof sodium sulfate per 100 cc.

3. An aqueous solution for chemically polishing articles formed of ametal consisting principally of aluminum and containing not more than 5per cent silicon comprising a concentrated aqueous phosphoric acidsolution containing 0.5-30.0 grams of sodium nitrate and 0.25-10.0 gramsof sodium sulfate dissolved in 100 cc. of 75 per cent phosphoric acid.

4. An aqueous solution for chemically polishing articles formed of ametal consisting principally of aluminum and containing not more than 5per cent silicon comprising a concentrated aqueous phosphoric acidsolution containing approximately 6 grams of sodium nitrate andapproximately 1.5 grams of sodium sulfate dissolved in 100 cc. of 75 percent phosphoric acid.

5. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan 5 per cent of silicon which comprises the steps of forming anaqueous bath consisting principally of phosphoric acid and containing0.5-30.0 grams of an alkali metal nitrate and 0.25-10.0 grams of analkali metal sulfate per 100 cc. of phosphoric acid, maintainingthe'temperature of the bath at 180-275 F. and bringing the surface ofthe laluhminum article to be polished into contact with the 6. Themethod for chemically polishing the surface of articles formed from ametal consisting principally of aluminum and containing not more than 5per cent of silicon which comprises the steps of forming an aqueous bathcontaining 0.5,30.0 grams sodium nitrate and 0.25- 10.0 grams of sodiumsulfate dissolved in 100 cc. of 75 per cent phosphoric acid, maintainingthe temperature of the bath at 180-275" F., and bringing the surface ofthe aluminum article to be polished into contact with the bath.

7. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan per cent of silicon which comprises the steps of forming an aqueousbath containing 0.S-30.0 grams sodium nitrate and 0.25-- 10.0 grams ofsodium sulfate dissolved in 100 cc. of 75 per cent phosphoric acid,maintaining the temperature of the bath at l50275 F., and bringing thesurface of ghehaluminum article to be polished into contact with the 8.The method for chemically polishing the surface of articles formed froma metal consisting principally of aluminum and containing not more than5 per cent of silicon which comprises the steps of forming an aqueousbath containing 0.5-30.0 grams sodium nitrate and 0.25- 10.0 grams ofsodium sulfate dissolved in 100 cc. of 75 per cent phosphoric acid,maintaining the temperature of the bath at l50275 F., bringing thesurface of the aluminum article to be polished into contact with thebath, dipping the article after treatment in a nitric acid bath andrinsing the article with water.

9. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan 5 per cent of silicon which comprises the steps of forming anaqueous bath containing 0.530.0 grams sodium nitrate and 0.25- 10.0grams of sodium sulfate dissolved in 100 cc. of 75 per cent phosphoricacid, maintaining the temperature of the bath at 150-275 F., bringingthe surface of the aluminum to be polished into contact with the bathfor not less than 1 nor more than 180 seconds, dipping the article aftertreatment in a nitric acid bath and rinsing the article with water.

10. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan 5 per cent of silicon which comprises forming an aqueous bath of05-300 grams sodium nitrate and 0.25-10.0 grams sodium sulfate dissolvedin 100 cc. 75 percent phosphoric acid, maintaining the temperature ofthe bath at about 210220 F., bringing the bath into contact with thealuminum surface to be polished for 20-60 seconds, rinsing the articlein dilute nitric acid and then rinsing the article in water.

11. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan 5 per cent of silicon which comprises forming an aqueous bath ofabout 6 grams sodium nitrate and about 1.5 grams sodium sulfatedissolved in 100 cc. per cent phosphoric acid, maintaining thetemperature of the bath at about 210- 220 F., bringing the bath intocontact with the aluminum surface to be polished for 20-60 seconds,rinsing the article in dilute nitric acid and then rinsing the articlein water.

12. The method for chemically polishing the surface of articles formedfrom a metal consisting principally of aluminum and containing not morethan 5 per cent of silicon which comprises forming an aqueous bath ofabout 6 grams sodium nitrate and about 1.5 grams sodium sulfatedissolved in cc. of 75 per cent phosphoric acid, maintaining thetemperature of the bath at about l50l80 F., bringing the bath intocontact with the aluminum surface to be polished for 5-50 seconds,rinsing the article in dilute nitric acid and then rinsing the articlein water.

References Cited in the file of this patent UNITED STATES PATENTS1,022,274 Strecker Apr. 2, 1912 2,409,271 Goldowski Oct. 15, 19462,428,464 Lum Oct. 7, 1947 2,434,021 Thompson Jan. 6, 1948 2,446,060Pray July 27, 1948

8. THE METHOD FOR CHEMICALLY POLISHING THE SURFACE OF ARTICLES FORMEDFROM A METAL CONSISTING PRINCIPALLY OF ALUMINUM AND CONTAINING NOT MORETHAN 5 PER CENT OF SILICON WHICH COMPRISES THE STEPS OF FORMING ANAQUEOUS BATH CONTAINING 0.5-30.0 GRAMS SODIUM NITRATE AND 0.2510.0 GRAMSOF SODIUM SULFATE DISSOLVED IN 100 CC. OF 75 PER CENT PHOSPHORIC ACID,MAINTIANING THE TEMPERATURE OF THE BATH AT 150*-275* F., BRINGING THESURFACE OF THE ALUMINUM ARTICLE TO BE POLISHED INTO CONTACT WITH THEBATH, DIPPING THE ARTICLE AFTER TREATMENT IN A NITRIC ACID BATH ANDRINSING THE ARTICLE WITH WATER.